FIGURE 2.

Mechanisms by which Hox factors could exhibit pioneer-like activity and alter chromatin accessibility. (A) Dfd and Abd-B Hox factors alone can bind to inaccessible chromatin and increase the chromatin accessibility of its targets without co-factor expression (Porcelli et al., 2019). (B) Pbx/Meis in vertebrates have been shown to bind inaccessible chromatin and promote chromatin opening. In this model, the Hox factor gains access to accessible DNA, forms a complex with Pbx/Meis, and this larger TF complex is required for accurate target regulation (Sagerstrom, 2004; Choe et al., 2014; Mariani et al., 2021). (C) Another potential model is that Hox TFs, a subset of which are capable of binding inaccessible DNA, recruit Exd/Pbx and Hth/Meis and together these complexes promote chromatin opening (Porcelli et al., 2019). Note that in each of these models, the role of the Hox factor in chromatin opening has not yet been confirmed.